The present disclosure relates to semiconductor memory devices. Due to their small-size, multifunctional, and/or low-cost characteristics, semiconductor devices are considered important elements in the electronics industry. Higher integration of semiconductor devices, such as semiconductor memory devices, may be beneficial to satisfy consumer demands for superior performance and inexpensive prices. In the case of semiconductor memory devices, increased integration may be especially beneficial, because their integration may be an important factor in determining product prices. However, the extremely expensive process equipment used to increase pattern fineness may set a practical limitation on increasing integration for semiconductor memory devices. To overcome such a limitation, a variety of studies on new technology for increasing integration density of a semiconductor device have been conducted.